Does anyone know off hand which oil has the highest HTHS value, of the 30 weights? Correct me if I'm wrong, but I think that specification would be beneficial in a supercharged engine (GM 3800) that is often driven aggressively.

I've noticed that the thicker GC 0w-30 I'm using now maintains oil pressure better (doesn't drop below half on the oil pressure gauge, 40 psi) when hot, compared to lighter oils such as PZ Ultra 5w-30, which often go down to 3/8 on the gauge. Whether that has any bearing (no pun intended :-) on protection or not, I'm unsure.

Would a thicker oil be of any benefit in this application? Perhaps something like M1 0w-40 or along those lines?

Does anyone know off hand which oil has the highest HTHS value, of the 30 weights? Correct me if I'm wrong, but I think that specification would be beneficial in a supercharged engine (GM 3800) that is often driven aggressively.I've noticed that the thicker GC 0w-30 I'm using now maintains oil pressure better (doesn't drop below half on the oil pressure gauge, 40 psi) when hot, compared to lighter oils such as PZ Ultra 5w-30, which often go down to 3/8 on the gauge. Whether that has any bearing (no pun intended :-) on protection or not, I'm unsure.Would a thicker oil be of any benefit in this application? Perhaps something like M1 0w-40 or along those lines?

Since you have an oil pressure gauge, (few vehicles do) you can optimize your oil viscosity selection by choosing the lightest oil (lowest HTHS vis') that still allows you to maintain the minimum OP as specified by the manufacturer. For most NA built GM engines that's nominally 50 psi at elevated rev's.There is zero lubrication benefit in using an oil that produces higher OP than required. So instead of looking for the thickest 30wt oil you should be looking for the lightest which probabily is M1 0W-30 (HTHS 3.06cP).The truth of the matter is, you could likely run a 20wt (HTHS 2.6cP) oil, and as long as your oil doesn't get too hot and your engine isn't prone to fuel dilution, you still won't likely test the OP minimum spec'. And since even the lightest 0W-20 oil is still way too heavy until oil temp's gets up to at least 75C you will enjoy superior overall lubrication.

The old oil lubrcation mantra applies in optimizing an oils viscosity; "as lightas possible, as thick as necessary". Follow that and you can't go wrong.

Yes, higher HTHS viscosity is definitely better for the bearings. See for example this ASTM white paper on the viscosity and bearing protection.You might simply use 5W-40, which should have an HTHS viscosity around 3.8.

Very misleading and poor advise.

A higher than necessary HTHS viscosity oil can be very bad for the bearings and in fact over heat them if the oil's HTHS vis is so high that the oil pump goes into by pass mode thereby reducing oil flow through the bearings with a resulting increase bearing temperature.

The spec' oil for NA built GM engines is a HTHS 3.1cP 30wt oil. There is no benefit to running anything heavier in tne 3.8L V6 or the 638hp supercharged ZR-1 Corvette engine.

Yes, higher HTHS viscosity is definitely better for the bearings. See for example this ASTM white paper on the viscosity and bearing protection.You might simply use 5W-40, which should have an HTHS viscosity around 3.8.

Very misleading and poor advise.

A higher than necessary HTHS viscosity oil can be very bad for the bearings and in fact over heat them if the oil's HTHS vis is so high that the oil pump goes into by pass mode thereby reducing oil flow through the bearings with a resulting increase bearing temperature.

The spec' oil for NA built GM engines is a HTHS 3.1cP 30wt oil. There is no benefit to running anything heavier in tne 3.8L V6 or the 638hp supercharged ZR-1 Corvette engine.

Your argument regarding oil pressure is seriously flawed. You will go into the bypass mode with any viscosity in high RPMs. You are also thinking about hydrostatic lubrication, not hydrodynamic lubrication, which is usually the case in internal-combustion engines. Oil pressure is not the only thing that protects the hydrodynamically lubricated engine parts, such as bearings. You need a thick enough (strong enough) oil film, which might not be the case with a smaller HTHS viscosity. Viscosity keeps moving parts away from each other, as the force exerted by the oil on the moving part is proportional to the viscosity.

Strictly speaking within the API classification system, RL 5w30 and 10w30 do meet the standards of kinematic viscosity at 100C and HTHS viscosity at 150C. The fact that their HTHS is 30% above the minimum set by the API does not disqualify them. API does not set maximum values for HTHS within a viscosity grade. (We may agree that this is archaic on API's part, but that's the rule.)

Also, ASTM-test authors found that HTHS viscosity at 150 C is not the only thing that is important for bearing protection but also the viscosity of the base oil is equally important. They say that less the SUS @ 100 F (such as 100N oil), more the critical lowest HTHS viscosity required to protect the bearings, or conversely more the SUS @ 100 F (such as 200N or 350N oil), less the critical lowest HTHS viscosity required to protect the bearings. See Page 5 of this white paper.

Therefore, there is no point in looking for a high-HTHS-viscosity 5W-30. Simply go to 5W-40 if you are doing high-speed driving and need more bearing protection. High HTHS viscosity alone is not sufficient -- you also need higher SAE viscosity, as your oil temperature hardly ever reaches 150 C.

Your argument regarding oil pressure is seriously flawed. You will go into the bypass mode with any viscosity in high RPMs. You are also thinking about hydrostatic lubrication, not hydrodynamic lubrication, which is usually the case in internal-combustion engines. Oil pressure is not the only thing that protects the hydrodynamically lubricated engine parts, such as bearings. You need a thick enough (strong enough) oil film, which might not be the case with a smaller HTHS viscosity.

Statements like this tell me you really don't have much if any practical understanding or knowledge of most IC engines.At normal operating temp's NO engine that I know of operates with the oil pump in by-pass mode at high rpm's when using the specified oil grade. Furthermore, the minimum (and optimum) oil pressure spec' that engine manufactures provide is well below the by-pass point. But if you think about it for a second it would make no sense to design an engine with the oil pump in by-pass mode at maximum rev's, thereby reducing oil flow when the engine needs it the most to cool the bearings.

Additionally your statement "oil pressure is not the only thing that protects the engine" implies a misunderstanding of hydrodynamic lubrication because oil pressure in itself doesn't protect an engine at ALL; in the bearings or elsewhere.The oil pressure provided by the oil pump is just to deliver the oil to every part of the engine that needs it and no more. It isn't that oil presuure that keeps the bearings apart, they do that all on their own in a running engine as long as sufficient oil is being supplied.When anyone talks about oil pressure as 92Saturn12 has, one is really talking about oil back-pressure which is what an oil pressure gauge actual reads. Lower oil back pressure is actual better because it indicates higher oil flow, and maximizing oil flow is what you want. Since HTHS viscosity correlates directly with oil pressure (kinematic viscosity does not) ideally you want the lowest HTHS vis that still provides the minimum oil backpressure as specified by the engine manufacturer.

You Sir by making the assertion that you want the "highest HTHS viscosity possible for maximum protection" are making the classic mistake of disasociating kinematic viscosity from HTHS viscosity when in fact they are inextricably linked. That's why if you control the oil temperatures you can maintain normal operating oil pressure when running very low HTHS viscosity oils with no increased engine wear. And by low I mean 5wt oil (HTHS 1.7cP) or lower. That's why in racing they can use 5wt qualifying oils, because oil temps are low. The engine doesn't know what the HTHS viscosity rating of the oil is all it knows is what the operational viscosity is and that is 100% temperature related.

I explained in greater detail the relationship between HTHS viscosity and kinematic viscosity in the following post:

CATERHAM, too much said about oil pressure and oil flow. Oil flow alone does not protect against wear either. Oil-film strength is more important, obviously as long as you have sufficient pressure and flow. I don't agree at all with the article "Dr. Haas" wrote on this site advocating the use of the thinnest possible oils, which has mislead many people on this forum. If thinnest oil was the solution for everyone, there would be no xW-30, xW-40, or xW-50 oils in the market.

By the way, most of us know the difference between the 100 C kinematic viscosity and 150 C high-shear viscosity (viscosity-index improvers make the oil a non-Newtonian, nonlinear fluid). Neither the 100 C viscosity nor the HTHS viscosity alone is a sufficient consideration on its own when choosing an oil.

Read the first five pages of the ASTM article I linked above. It's based on fact and testing, not on thinking. It clearly states that both the higher SAE viscosity and the higher HTHS viscosity together offer better wear protection. This is not to say that you should use the highest SAE viscosity and HTHS viscosity out there but choose it carefully based on your application (manufacturer recommendation, driving conditions, conditions of the engine [new, old, etc.], etc.).

It's amazing that people believe the word of the mouth (such as the "Dr. Haas" article) but ignore ASTM research articles.

Dr. Haas' intro' to motor isn't called Motor Oil 101 for nothing.There is no discussion of how viscosity is measured in an operating IC engine but that's fine; too much info' too soon is just confusing.But the gist of what he say's in fundamentally true; even the lightest motor is too thick when cold and one only needs a thicker than normal oil (a 30wt oil for simplicities sake) if higher than normal oil temp's are encountered. The objective is to maintain an optimal operational viscosity.

So what is the optimal operational viscosity for any application?The SAE grade on a bottle of oil won't tell you nor will knowing the particular viscosity spec's of a given oil.One really must rely on what the engine manufacturer recommends and as starting point is to use the recommended oil. Deviating from that recommendation should only be done with the utmost caution.If one does want to fine tune the oil's viscosity to one's own application the only way to do so is with reference to the manufacturer's oil back-pressure specifications. You need to install an oil pressure gauge to do this if your car doesn't come equipped from the factory with one.

This instrument serves many functions including that of an on board viscometer. If the mfter's spec' OP is say 50 psi @ 5,000 rpm then you should maintain that under all conditions no matter how hot the oil gets, how much the oil has sheared or has been diluted with fuel. If with experience you notice that your OP never ever drops below 60 or 65 psi then you could rightfully question whether the spec' oil you're running might be too thick. If you then tried a one grade lighter oil (an oil with a lower HTHS vis to be specific) and then observed your oil pressure now was in the 55 psi range you're still complying with the min' OP spec's for the engine. There will be many benefits to using the lighter oil such as less start-up wear when the oil is cold and way thicker than necessary, an improvement in fuel economy and engine power due to less power robbing oil drag.

But lets say you've heard that thicker oil (a higher HTHS vis than spec') will provide more engine protection because of it's inherently higher film strength so you want to try that.So with this thicker oil your hot OP now is up to 70 psi isn't that better? But wait a minute, you've noticed that your OP never is higher than 70 psi because that is the by-pass point of your oil pump. Have you gained any benefit from running this thicker, higher film strength oil? The answer is no. Since you can't use maximum rev's without the oil pump diverting oil from passing through the engine, it will now run hotter than the mft intended; not a good thing and therefore you cannot benefit from the increased film strength of the thicker oil.

When a mft has done all the hard work of determining the minimum optimum OP for an engine and therefore the minimum operational viscosity is there any benefit from deviating from that recommendation? Not that I know off and in fact I have found the mft's spec' to be quite conservative. I know a lot of situations where sports cars have been tracked and gotten the oil temp's very high indeed on the spec' oil, to the point where the OP has dropped below the mft's spec' minimum under high stress racing conditions with no subsiquent evidence of bearing wiping. And usually running a heavier than spec' oil to deal with the lower OP under high oil temp' conditions doesn't result in better UOA's. Having said that, I believe it is still prudent to maintain the minimum OP spec's particularly under sustained conditions.

Why do vehicle manufacturers specify thicker grades of oils in other countries for the same vehicle if there is a disadvantage to it as you suggested?

A lot of vehicles sold in other countries recommend a 5w-40 or equivalent for vehicles that specify 5w-20 or 5w-30 here. I'm not drawing any conclusions based on that fact, just asking the question why.